The combustion of natural gas (and other low-molecular-weight gaseous fuels including oil aerosols) to generate other forms of energy is one of the cleanest methods of combustion-based energy conversion. However, in any combustion process, some drawbacks are present. One of the most serious problems is the generation of pollutants, such as oxides of nitrogen. It is well established that nitrogen oxides are a source of ozone generation in the atmosphere and that ozone, along with unburned hydrocarbons, leads to the eventual formation of other components of photochemical smog, such as peroxyacyl nitrates. Current governmental regulations are directed at reducing the NO.sub.x levels that are released into the atmosphere, in an attempt to reduce the occurrence of photochemical smog.
To accommodate these regulations, research has been directed at the development of more efficient burners and catalysts. One group of compounds which have been found to reduce NO.sub.x formation, when low-molecular-weight fuels are burned at high temperature, are the perovskite-type ceramic oxides. These compounds have been shown to reduce the formation of nitrogen oxides during combustion and will be referred to as "superemitting" ceramics. Such materials often have an element present in a mixed oxidation or mixed valence state, forming a nonstoichiometric oxide. Some of the most effective members of this class of compounds are rare earth/alkaline earth oxide systems, rare earth/transition metal oxide systems, and various other mixed metal oxide systems.
Superemitters, when heated to a threshold temperature, emit visible or infrared radiation. Such radiation can be absorbed by a photovoltaic device, such as a silicon cell, to produce output voltage or current. The thermally-stimulated superemitters produce radiation in a relatively concentrated, narrow spectral band compared to blackbody or "grey body" emitters, which typically exhibit a broad band thermal emission. As a result of the concentrated, narrow spectral band, the power generated by a superemitter is greater than that generated by a blackbody emitter.
The thermophotovoltaic device used to collect the radiation is designed so that the superemitter emits radiation of wavelength near the photovoltaic material band gap. For example, silicon has a band gap at about 1,100 nm, and InGaAsP and has a band gap at about 1,300 nm. An ytterbia-based, mixed oxide emission spectrum is compared with that of holmium oxide and with a typical blackbody spectrum in FIG. 1.
Although general fiber matrix burners have been developed, they have generally not been found to be effective at high temperatures to function at over 650,000 watts/m.sup.2, producing selected band(s) of radiation as well as producing low NO.sub.x levels, i.e., less than about 20 ppm. Some such burners are capable of operation at a nitrogen oxide emission level of 20 to 30 ppm below 650,000 watts/m.sup.2 in both laboratory and field tests. However, when the energy density of such burners is increased to above 650,000, they deteriorate and NO.sub.x emissions increase dramatically.
Therefore, there is a need for a high-energy density burner that produces low NO.sub.x emissions when operating at a high-energy density.
Superemitter ceramic burners, which emit radiation in a narrow spectrum when heated above their threshold temperature offer the potential for such high-efficiency energy production. It is desirable that these superemitter ceramic burners have highly active emissive surfaces and low NO.sub.x and other combustion products. It is also desirable that these superemitters be inexpensive and easy to produce, strong and durable, and have high-temperature and high-energy density capabilities. The intensity of the light emitted from a superemitter increases dramatically with temperature. Therefore, the amount of radiant energy emitted and then collected by the photovoltaic cell will also increase dramatically with temperature.
It is also desirable to produce electric power efficiently. The efficiency of the electric power goes up if the energy in the exhaust gas is recycled by means of a recuperator, which transfers the energy in the exhaust to the air inlet. The recuperator may increase the temperature of the air above the autoignition point. To provide for this important energy feature, a fuel injection system has been invented that allows combustion inlet temperature to reach well over autoignition.
Therefore, there is a need for an improved fiber matrix burner technology for a wide variety of applications in heating, electrical energy generations, cooking, and providing photons of specific wave bands for such purposes as pumping lasers and operating photochemical reactors.